ISO code modding, Tenchu TOA.

Hey guys, new to the forum and have a bit of a tough question.
I'm curious as to what length PSP game code can be altered and how one might go about such a thing. I've seen mods for FF Dissidia and 012 and other games for extra character skins to total redesigns of aspects of gameplay and soundtrack/texture alteration so I would assume it can be done with core game code as well.

To put context on this, I'm looking at modifying the game Tenchu: Time Of The Assassins on PSP to fix that abhorrent draw distance/fog of war, whatever you want to call it.
The game is brilliant, but this one issue renders it unplayable for less experienced players/Tenchu fans.
I personally can't believe the game was released with such a critical flaw so I'd like to try and fix it, specifically for my copy.

Truthfully I don't know sweet stuff all about coding on any level, but I've been told that with a combination of programs like UMDgen and ISObuster that it should be possible and I'm willing to do any amount of research necessary to fix this game.
I also plan to work on fanmade Tenchu titles in the near future so I believe learning this sort of thing is necessary.